SAN FRANCISCO, Sept. 5 — A consortium of technology companies, including I.B.M. and Cisco Systems, announced plans Tuesday for a vast wireless network that would provide free Internet access to big portions of Silicon Valley and the surrounding region as early as next year.

The project is the largest of a new breed of wireless networks being built across the country. They are taking advantage of the falling cost of providing high-speed Internet access over radio waves as opposed to cable or telephone lines.

The project will cover 1,500 square miles in 38 cities in San Mateo, Santa Clara, Alameda and Santa Cruz Counties, an area of 2.4 million residents. Its builders, going by the name Silicon Valley Metro Connect, said the service would provide free basic wireless access at speeds up to 1 megabit a second — which is roughly comparable to broadband speeds by telephone — in outdoor areas. Special equipment, costing $80 to $120, will be needed to bolster the signal enough to bring it inside homes or offices.

The consortium will also offer a fee-based service, with higher speeds and technical support, and will allow other companies to sell premium services over the network as well.

Diana Hage, director of wireless services at I.B.M., said she expected the project to cost $75 million to $270 million. She said the project was meant to be a public service and, by showing the potential for the technology, to develop and promote the companies’ commercial interests.